1. Field of the Invention
The present invention relates to a process for electrograining an aluminum substrate for lithographic printing.
2. Description of the Prior Art
In general, when an aluminum substrate is used as a lithographic plate, the surface is grained beforehand to improve the adhesion of the subsequently applied light-sensitive coating and to improve water retention in the non-image areas during printing. Such graining conspicuously affects the printability and durability of the plate for offset printing, and the quality of the graining is one of important factor in producing effective plates.
Aluminum substrates are conventionally grained for lithographic printing by mechanical graining, such as ball-graining and slurry brushing, or by electrograining. Electrograining, i.e., electrochemical etching in an acidic solution, has become attractive in recent years, because it is suitable for treating not only aluminum sheets cut to a length but continuous strips.
In the electrograining, alternating current is passed between two aluminum plates or sheets facing each other or between an aluminum plate and a suitable counter electrode, such as a graphite plate, in an electrolytic cell containing an electrolyte, the main or sole solute of which is hydrochloric acid or nitric acid. When the electrolyte is mainly nitric acid, the grained surface obtained has relatively finely pitted structure, and shows the so-called "pits-within-a-pit" structure, i.e., the surface is formed of fine pits, which themselves contain many finer pits. However, the depth of the pits is generally shallow. In contrast, when the electrolyte is mainly hydrochloric acid, the depth of the pits is generally deep, but the surface of an individual pit is relatively smooth, and does not exhibit the complex graining as occurs when an electrolyte of nitric acid is used.
Such differences in the topography of the grained surface delicately affects the printability and durability of the plate, thus limiting their application. The substrate grained in an electrolyte of nitric acid is used mainly to produce a plate for relatively short run commercial printing involving delicate printed matter. On the other hand, the substrate grained in an electrolyte of hydrochloric acid is used mainly to produce a plate for long run printing of newspapers, magazines, etc., in which reproduction of delicate images is not required.
Furthermore, it is a common problem in the conventional electrograining process using conventional alternating current, that the electrolyte composition considerably restricts the electrograining conditions to achieve uniform graining, thus limiting the resultant topography and pit size within narrow ranges.
After extensive study, it has been found that the topography and pit size can be varied without imparing grain uniformity by independent control of both anodic and cathodic reactions, and that this can be accomplished by using "regulated alternating current." The phrase "regulated alternating current" as used in the present invention indicates an electric current in which the anodic voltage and the cathodic voltage as well as duty cycle are respectively independently regulated in contrast to conventional AC. When an aluminum substrate for lithographic printing is electrograined using as electrolyte either of hydrochloric acid or nitric acid, a uniformly and finely grained substrate with "pits-within-a-pit" structure can be efficiently obtained within a short time, by using regulated alternating current, which is characterized by applying an inter-electrode voltage in which the anodic voltage (V.sub.A) is arranged to be higher than cathodic voltage (V.sub.C), thereby adjusting anodic coulombic input (Q.sub.A) to be greater than cathodic coulombic input (Q.sub.C). The diameter and depth of the pits can be optionally adjusted by properly selecting the ratio of cathodic coulombic input to anodic coulombic input (Q.sub.C)/(Q.sub.A) given by the voltage adjustment.
The object of the present invention is to provide a process for electrograining an aluminum substrate for lithographic printing in which the aluminum substrate is electrograined in an electrolytic cell using an electrolyte of hydrochloric acid or nitric acid with regulated alternating current to apply interelectrode voltage with anodic voltage (V.sub.A) arranged to be higher than cathodic voltage (V.sub.C).